When importing with a reference, only require 1 beam model if use_beam_reference=True

[jbeilstenedmands]

When importing with a reference for stills data, if we don't want to use the beam reference then we should not need to assert that the beam models have been combined.

[codecov]

Codecov Report

Merging #2345 (4014e77) into main (621c781) will increase coverage by 2.34%.
The diff coverage is 93.95%.

❗ Current head 4014e77 differs from pull request most recent head 914df65. Consider uploading reports for the commit 914df65 to get more accurate results

@@            Coverage Diff             @@
##             main    #2345      +/-   ##
==========================================
+ Coverage   80.72%   83.07%   +2.34%     
==========================================
  Files         589      593       +4     
  Lines       68175    68542     +367     
  Branches     9145     9213      +68     
==========================================
+ Hits        55036    56943    +1907     
+ Misses      11031     9468    -1563     
- Partials     2108     2131      +23     

[jbeilstenedmands]

I made an extra change to rearrange the order of assertions. This means that you can in theory use a reference experiment with solely a detector model (if use_beam_reference=False and use_gonio_reference=False).